J. Chem. Sci. Vol. 127, No. 5, May 2015, pp. 897–908. c© Indian Academy of Sciences.
DOI 10.1007/s12039-015-0848-4
Understanding the surface and structural characteristics of tungstenoxide supported on tin oxide catalysts for the conversion of glycerol
M SRINIVASa, G RAVEENDRAa, G PARAMESWARAMa, P S SAI PRASADa,
S LORIDANTb and N LINGAIAHa,∗
aCatalysis Laboratory, I & PC Division, CSIR-Indian Institute of Chemical Technology,
Hyderabad 500 007, IndiabInstitut de Recherchessur la Catalyse et l’Environnement de Lyon, IRCELYON, UMR 5256, CNRS,
Université Lyon I, 2 avenue Albert Einstein, 69626 Villeurbanne Cedex, France
e-mail: [email protected]
MS received 28 August 2014; revised 1 December 2014; accepted 28 December 2014
Abstract. Catalysts with varying WO3 content on SnO2 were prepared and characterized by X-ray diffrac-tion, in situ Raman spectroscopy, X-ray photoelectron spectroscopy and temperature programmed desorptionof NH3. In situ Raman analysis reveals the presence isolated monomers and polymeric species of WO3. Thesecatalysts were evaluated for the conversion of glycerol into value added chemicals. Etherification of glycerolwith tertiary butanol and preparation of glycerol carbonate from glycerol and urea are studied over these cata-lysts. The catalytic activity results suggest that the glycerol conversion and selectivity depends on the morphol-ogy of WO3 which in turn is related to its content in the catalyst. The catalysts with 5 wt.% of WO3 on SnO2
resulted in high dispersion with larger number of strong acidic sites. The selectivity in the glycerol etherifica-tion is related to the nature of the catalyst and reaction time. These catalysts also exhibited high activity forsynthesis of glycerol carbonate. The effect of various reaction parameters was studied to optimize the reactionconditions. The catalysts also exhibited consistent activity upon reuse.
Keywords. Tungsten oxide; tin oxide; etherification; glycerol; tert-butanol; glycerol ethers
1. Introduction
Biodiesel, an eco-friendly biofuel is being chosen as an
alternative to fossil fuels. Biodiesel has been produced
in large quantities by transesterification of vegetable
oils or animal fats.1 During the production of biodiesel
approximately 10 wt.% of glycerol is obtained as by-
product.2 As the production of biodiesel is increasing
with time, large quantities of glycerol have been accu-
mulated in an already saturated market. This has led to
a dramatic reduction on the economic value of glycerol
as a raw material. This situation has fuelled a growing
interest to look for new glycerol uses.3 Glycerol can
be converted into several important chemicals4–8 such
as 1,2-propanediol, 1, 3- propanediol, acrolein, glyceric
acid, esters of glycerol and syngas, etc. The details
about the glycerol conversion are comprehensively dis-
cussed in recent reviews.9–11 Etherification of glycerol
with isobutylene or tert-butanol towards fuel oxygena-
tes has been considered recently as a promising and
economically viable method for the conversion of glyc-
erol. Another important chemical derived from glycerol
∗For correspondence
is glycerol carbonate (GC), a highly value added product
with many potential applications.12–16 Etherification of
glycerol is generally facilitated by acid catalysts. Thus,
acid catalysts such as sulfuric acid, p-toluenesulfonic
acid, acidic ion exchange resins, wide pore zeolites, and
acid-functionalized mesostructured silicas have been
used for etherification of glycerol.17–27 Etherification
of glycerol with tert-butanol produce a mixture of mono-
tert-butyl glycerol ether (ME), di-tert-butyl glycerol
ether (DE), and tri-tert-butyl glycerol ether (TE). Selec-
tive etherification of glycerol into monoalkyl glycerol
ethers, which exhibit a wide spectrum of biological
activities such as anti-inflammatory, anti-bacterial, anti-
fungal, immunological stimulation and anti-tumour
properties.28 Another important derivative of ME, diox-
olanes, is also used as co-fuels for the diesel fraction.20
The main routes proposed for the synthesis of GC
are: (i) reaction of glycerol with phosgene,29 (ii) trans-
esterification of glycerol with dimethyl carbonate30–32
or ethylene carbonate,33 (iii) reaction of glycerol with
urea,33–36 and (iv) carbonation of glycerol with carbon
dioxide.37,38 Among the possible routes for the prepara-
tion of GC, the reaction of glycerol and carbon dioxide
in the presence of a catalyst is the best approach.
897
898 M Srinivas et al.
However, this method requires severe reaction condi-
tions and the yield of glycerol carbonate is too low
to be used for practical purpose.37,38 An alternative
route for the GC synthesis is carbonation of glycerol
with dimethyl carbonate or urea. Several authors stud-
ied the performance of solid catalysts including ZnO,39
zinc sulfate,40 ZnCl2,41 γ –zirconium phosphate,42
HTc–Zn derived from hydrotalcite,43 Co3O4/ZnO,44
Sm exchanged heteropoly tungstate,45 gold supported
ZSM–5,46 manganese sulfate,46 etc., for the preparation
of GC from glycerol and urea. Although useful catalysts
are reported, it is still a challenging task to develop
new heterogeneous catalysts having a high performance
under milder conditions.
Tin oxide (SnO2) is one of the most attractive func-
tional materials because of potential applications par-
ticularly as a catalyst and also as a carrier for supported
catalysts. Furuta et al.47 compared the activities of sul-
fated zirconia and sulfated tin oxide for etherification
reaction of methanol and reported that sulfated tin oxide
is active due its high acidity. However, in order to pro-
mote the thermal stability and catalytic property of the
pure tin oxide, using high valence cation of oxide was
previously reported.48,49 Thus, it is of interest to study
the novelties of WO3 supported SnO2 catalysts for the
conversion of glycerol.
In the present study, a series of WO3 supported on
SnO2 catalysts were prepared and studied for selective
etherification of glycerol with tert-butanol and synthe-
sis of glycerol carbonate from glycerol and urea. Under-
standing the surface and structural properties of the
catalysts towards glycerol etherification and optimiza-
tion of reaction parameters was also the aim of the
present study.
2. Experimental
All the chemicals used in this study were of analytical
grade. Glycerol (99%) was obtained from Fisher Sci-
entific. tert-butanol (99%), ammonium meta-tungstate
(99.5%) and tin oxide (99.9%) were obtained from
Sigma-Aldrich. A series of WO3/SnO2 catalysts were
prepared by impregnation method. In a typical method
required amount of ammonium meta-tungstate hydrate
[(NH4)6H2W12O40. nH2O] was dissolved in distilled
water and this solution was added to calculated amount
of tin oxide with stirring. The solution was stirred for
a period of 2 h and the excess water was removed on a
hot plate. The dried sample was kept overnight for fur-
ther drying at 393 K. The final catalyst was obtained by
calcination at 773 K for 4 h. The amount of WO3 on
SnO2 is varied from 2 to 10 wt. %. The catalysts were
designated as x% WS, where x indicates the weight per-
centage of WO3 on SnO2 and W and S stands for WO3
and SnO2.
2.1 Characterization of the catalysts
X-ray diffraction (XRD) patterns of the catalysts were
recorded on a RigakuMiniflex diffractometer using
CuKα radiation (1.5406 A◦) at 40 kV and 30 mA. The
measurements were obtained in steps of 0.045◦ with an
account time of 0.5 s and in the 2θ range of 10–80◦.
Confocal micro-Raman spectra were recorded with a
LabRam HR Raman spectrometer (Horiba-JobinYvon)
equipped with BXFM confocal microscope, interfer-
ence and Notch filters and Charge-Coupled Device
detector. The exciting lines at 514.5 nm of a 2018 RM
Ar+–Kr+ laser (Spectra Physics) were focused using
×50 long working distance objective and the diffused
light was dispersed with grating of 1800 lines−1.mm−1
leading to spectral resolution of ca 0.5 cm−1. In situ
Raman spectra were recorded between RT and 873 K
under air using THMS600 cell (Linkam). The homo-
geneity of the catalysts at the micrometer scale was
controlled achieving spectra on different points.
Temperature programmed desorption (TPD) of
ammonia was carried out on a laboratory-built appara-
tus equipped with a gas chromatograph using a TCD
detector. In typical experiments about 0.1 g of the oven
dried sample was taken in a quartz tube. Prior to TPD
studies, the catalyst sample was treated at 573 K for 1
h by passing pure He gas (99.995%, 50 mL/min). After
pre-treatment, the sample was saturated with anhydrous
ammonia (10% NH3 balance He) at 373 K with a flow
rate of 50 mL/min for 1 h and was subsequently flushed
with He at the same temperature to remove physisorbed
ammonia. The process was continued until a stabilized
base line was obtained in the gas chromatograph. Then
the TPD analysis was carried out from ambient temper-
ature to 973 K at a heating rate of 10 K/min. The amount
of NH3 evolved was calculated from the peak area of
the already calibrated TCD signal.
X-ray photo electron spectroscopy (XPS) measure-
ments were conducted on a KRATOS AXIS 165 with
a DUAL anode (Mg and Al) apparatus using Mg Kα
anode. The non-monochromatized AlKα X-ray source
(hν = 1486.6 eV) was operated at 12.5 kV and 16 mA.
Before acquisition of the data, each sample was out-
gassed for about 3 h at 373 K under vacuum of 1.0
× 10−7 Torr to minimize surface contamination. The
XPS instrument was calibrated using Au as standard.
For energy calibration, the carbon 1s photoelectron line
was used. The carbon 1s binding energy was taken as
285 eV. Charge neutralization of 2 eV was used to
Catalytic conversion of glycerol to ethers 899
Scheme 1. Etherification of glycerol with tert-butanol.
balance the charge up of the sample. The spectra were
decomposed into individual components using Sun
Solaris Vision-2 curve resolver. The location and the
full width at half maximum (FWHM) value for the
species were first determined using the spectrum of a
pure sample. Symmetric Gaussian functions were used
in all cases. Binding energies for identical samples
were, in general, reproducible within ±0.1 eV.
2.2 Reaction procedures
2.2a Etherification of glycerol with tert-butanol:
Etherification of glycerol with tert-butanol (scheme 1)
was carried out in a 100 mL haste alloy PARR auto-
clave. Required quantities of glycerol, tert-butanol and
catalyst were introduced in to the autoclave and purged
three times with N2. After the third purge, the reactor
was heated to desired reaction temperature. During
the course of reaction, pressure was developed autoge-
nously. After the reaction, the gas phase products were
collected in a gasbag and the liquid phase products were
separated from the catalyst by filtration. The liquid sam-
ples were analyzed by using a gas chromatograph (Shi-
madzu, 2010) equipped with flame ionization detector
(FID) using INNO-WAX capillary column (diameter:
0.25 mm, length 30 m). The products were confirmed
by GC–MS (Shimadzu, GCMS-QP2010) analysis. The
gas products were analyzed by using a gas chromato-
graph (Porapak Q column) equipped with a thermal
conductivity detector (TCD). The products identified
during etherification of glycerol were mono tert-butyl
glycerol ethers (ME), di tert-butyl glycerol ethers (DE),
and tri tert-butyl glycerol ethers (TE).
2.2b Carbonylation of glycerol with urea: Carbony-
lation of glycerol with urea (scheme 2) was performed
in a 25 mL two neck round-bottom (RB) flask under
reduced pressures. In a typical experiment glycerol (2
g), urea (1.306 g) and catalysts (0.2 g) were taken in
the round bottom flask and heated in an oil bath at
413 K with constant stirring. One neck of the RB flask
was connected to vacuum line. Reaction was run under
a reduced pressure in order to remove the ammonia
formed during the reaction. After completion of reac-
tion or stipulated time, methanol was added and the
catalyst was separated by filtration. The products were
identified by GC–MS (Shimadzu, GCMS-QP2010S)
analysis. The products were analyzed with a Shimadzu,
2010 gas chromatograph equipped with INNO-WAX
capillary column (diameter: 0.25 mm, length 30 m) and
flame ionization detector.
Conversion of the glycerol and selectivity was calcu-
lated on the basis of the following equation with respect
to glycerol:
Conversion (%) =Number of moles of glycerol reacted
Total number of moles of glycerol taken×100
Selectivity (%) =Number of moles desired product
Total number of moles of all products×100
3. Results and Discussion
3.1 X-ray diffraction of catalyst
XRD patterns of the catalysts are shown in figure 1. The
patterns of the SnO2 support and WO3 were included
for the sake of comparison. The XRD pattern were
quite consistent with the tetragonal cassiterite (rutile)
(JCPDS No. 41-1445) structure of SnO2,38 and no char-
acteristic diffraction peaks of the impurities and other
compounds were found with the lower loadings of
Scheme 2. Reaction scheme for the carbonylation of glycerol with urea.
900 M Srinivas et al.
Figure 1. XRD patterns of x% WO3/SnO2 catalysts (a)2WS (b) 5WS, (c) 7.5WS, (d) 10WS and (e) WO3.
WO3, this implies that the WO3 (≤5 wt.%) might exist
as an amorphous oxide on the SnO2, and is considered
to act as a barrier against development of grain bound-
aries of SnO2 and effectively prevent the grain growth.50
When the composition of the WO3 in the catalyst was
higher than 5 wt.%, the diffraction peaks of monoclinic
WO3 crystals were observed in the XRD patterns.51,52
The crystallization of WO3 was accompanied by a loss
of specific surface area.
3.2 In situ Raman spectra
In situ Raman spectra were achieved during calcina-
tion up to 873 K under air of freshly prepared 2, 5
and 7.5WS catalysts. Raman spectra of these catalysts
were measured with stepwise increase in temperature.
The most relevant spectra of 2, 5 and 7.5WS catalysts
are shown in figures 2(a), 2(b) and 2(c) respectively.
200 400 600 800 1000 1200
873 K
723 K
773 K
573 K
673 K
473 K
RT
Inte
ns
ity
(a
.u)
Raman Shift (cm-1
)
Raman Shift (cm-1
)
Raman Shift (cm-1
)
200 400 600 800 1000 1200
873 K
823 K
773 K
748 K
723 K
698 K
673 K
573 K
473 K
RT
Inte
ns
ity
(a
.u)
(b)
200 400 600 800 1000 1200
873 K
773 K
723 K
673 K
573 K
473 K
RT
Inte
ns
ity
(a
.u)
(c)
(a)
Figure 2. In situ Raman spectra of x% WS catalysts (a)2WS, (b) 5WS and (c) 7.5WS
Catalytic conversion of glycerol to ethers 901
The 2WS catalyst showed predominantly the bands
at 475, 629 and 768 cm−1 related to SnO2 cassi-
terite crystalline phase. The band at 685 cm−1 was
assigned to an A2u (LO) mode that becomes weakly
Raman-active because of structural distortions induced
by disorder.53,54 Additionally, the broad bands at 850
and 985 cm−1 were respectively attributed to stretch-
ing vibrations of W-O-W bridging bonds (ν(W-O-W))
and to stretching vibrations of terminal W=O bonds
(ν(W=O)) of hydrated polymeric by analogy with the
spectra of such species supported over other oxides.55,56
The bands near 270, 715 and 805 cm−1 related to crys-
talline α-WO357,58 were absent even after the catalyst
exposed to 873K. This suggests that the WO3 was well
dispersed on SnO2. Upon raising the temperature up
to 873 K, the ν(W=O) vibrations were shifted to 998
cm−1 and became thinner because of dehydration55–57
whereas the ν(W-O-W) vibrations around 850 cm−1
remained almost unchanged. The Raman spectra of
5WS catalysts showed similar spectra as that of 2WS
at low calcination temperatures. Above 673 K, a thin
(W=O) band at 999 cm−1 and a broad one at 1017 cm−1
were distinguished and ascribed to isolated monomers
and polymeric species respectively.56,59 Broad bands
near 715 and 805 cm−1 that remained weak from 673 to
773 K were additionally observed and were ascribed to
WO3 nano particles. The micro-Raman spectra obtained
on different areas of the sample revealed good homo-
geneity at the micrometre scale which suggested that
these nano particles were well dispersed on SnO2.
Above 773 K, the bands of crystalline α-WO3 became
intense revealing crystalline growth and the micro-
Raman spectra achieved on different areas showed the
heterogeneities at the micro meter scale. The Raman
spectrum of uncalcined 7.5WS catalyst was differ-
ent from the spectra of 2 and 5WS catalysts. Indeed,
in addition to the bands of SnO2, some bands were
observed at 975, 960 and 935 cm−1 and attributed to
(NH4)6H2W12O40 salt54 that re-precipitated during dry-
ing because of the high concentration of the impreg-
nation solution. Additionally, formation of crystalline
WO3 was evidenced above only at 673 K. This fea-
ture was related to the higher WO3 surface density
at which the monolayer coverage was reached. These
results indicated that the maximal coverage of WO3
clusters on SnO2 was obtained for the 5WS catalyst
after calcination at 773 K which is in agreement with
XRD observations.
3.3 TPD of ammonia
Ammonia adsorption–desorption technique permits to
determine the strength of acid sites present on catalyst
373 473 573 673 773 873 973
(e)
(d)
(c)
(b)
Inte
ns
ity
(a
.u)
Temperature (K)
(a)
Figure 3. Temperature programmed desorption profiles ofammonia on WS catalysts (a) SnO2, (b) 2WS, (C) 5WS, (d)7.5WS, and (e) 10WS
surface, together with total acidity. The TPD profiles
of the catalysts are shown in figure 3. The TPD pro-
file of SnO2 sample was also included for sake of com-
parison. The catalysts with low WO3 (2 WS) content
did not show any appreciable acidity generated by the
presence of molecular tungsten oxide. However, with
further increase in WO3 content, a drastic enhancement
in the acidity was noticed. This catalyst showed NH3
desorption peaks centred between 743 K and 973 K.
The high temperature desorption of NH3 corresponds
to moderate to strong acidic sites of the catalysts orig-
inating due to well-dispersed WO3 clusters. After fur-
ther increasing the WO3 content, the high temperature
desorption peak shifted to low temperature. The cat-
alysts with high WO3 amount showed only one des-
orption peak at high temperature that was associated
to the crystalline WO3 present on SnO2 as revealed by
XRD patterns and Raman spectra. The acidity values
of these catalysts are presented in table 1. As a whole
the TPD results suggests the presence of well-dispersed
WO3 clusters on SnO2 leads to generation of maximum
number moderate to strong acidic sites.
Table 1. Acidity and surface area of the catalysts.
S. No. Reaction time (min) SBET(m2/g) Acidity (mmol/g)
1 SnO2 11.6 0.0642. 2WS 10.5 0.2103. 5WS 9.8 0.2714. 7.5WS 8.4 0.1585. 10WS 7.1 0.1266. WO3 6.0 0.135
902 M Srinivas et al.
3.4 Activity measurements of the catalysts
3.4a Etherification of glycerol: The etherification of
glycerol was carried over WO3/SnO2 catalysts and the
results are provided in figure 4. The etherification of
glycerol over these catalysts leads to the formation of
mainly mono tertiary butyl glycerol ether with selectiv-
ities reaching ca 88%. A small amount of both di and tri
tertiary butyl glycerol ethers are also formed. There was
no much variation in the selectivities of the catalysts
with change in WO3 loading. Additionally, the figure
clearly show that the rate of the reaction increased with
increase in the WO3 loading up to 5 wt.% and thereafter
decreased. The optimum activity obtained for the cata-
lyst with 5% WO3 on SnO2. The catalytic reaction rate
Vt can be calculated by the following equation:
Rate of the reaction, Vt =ms
t Mcat
Where vt is the catalytic reaction rate, ms is the moles
of the glycerol converted within the reaction time of t ,
and Mcat is the mass of a catalyst.
In order to investigate the variation in activity and
selectivity of the 5WS catalyst with reaction time, data
were collected at different reaction times and the results
are shown in table 2. The glycerol conversion increased
with reaction time from 30 to 60 min and thereafter no
much variation was noticed. The conversion of glyc-
erol was high even at 30 min suggesting the high activ-
ity of the catalyst. The overall selectivity to ME is
more. The formation of higher tertiary butyl glycerol
ether (HE) was noticed at longer reaction times. Ini-
tially, the selectivity towards mono ether was high and
2.5 5.0 7.5 10.0
0
20
40
60
80
100
WO3 content (%)
Sele
cti
vit
y (
%)
1.5
2.0
2.5
3.0
3.5
4.0
Conversion of glycerol
ME
DE
TE
Rate
of
the r
eacti
on
(1
0-2 m
ol/g
-h)
Figure 4. Catalytic performance of WS catalysts for thesynthesis of mono tert-butyl glycerol ether. Reaction condi-tions: glycerol (1.84 g),tert-butanol (13.34 g), catalyst weight(0.5g), reaction temperature (393 K), reaction time (1 h).
Table 2. Comparison of glycerol conversion and productselectivity with reaction time.
Reaction Time (Min) Conversion (%) Selectivity (%)ME DE TE
30 68.6 94.5 3.8 1.760 88.8 87.8 7.5 4.790 90 87.1 6.6 6.3120 82.2 85.8 7.7 6.5
Reaction conditions: glycerol (1.84 g), tert-butanol (13.34 g),catalyst weight (0.5 g), reaction temperature (393 K).
as the reaction prolonged, the selectivity towards di and
tri ethers increased marginally at the expense of mono
ether. Overall the catalyst is highly active and selective
towards ME within short reaction time.
The activity of the catalysts can be explained based
on their surface and structural characteristics. The XRD
patterns suggest the presence of amorphous WO3 phase
for the catalysts with low WO3 content. The formation
of WO3 crystallites were observed for the catalysts with
>7.5WO3. The high activity of 5WS catalysts might
be due to the presence of well-dispersed WO3 clusters
on SnO2 surface. This catalyst also showed maximum
acidity with high number of moderate and strong acidic
sites. Similar behaviour is reported for WO3 based cat-
alysts where the presence of amorphous WO3 on ZrO2
resulted in generation of strong acidic sites and thereby
high esterification activity.60 The acidity of the catalysts
decreased with the presence of crystallites of WO3.
The 5WS catalyst showed the presence of more
number of acidic sites (table 1) which are responsi-
ble for high activity figure 5. Ball et al.61 reported that
the reaction between alcohols with urea to form alkyl
673 773 873 973
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Rate of reaction
Acidity of the catalysts
Temperature (K)
Ra
te o
f th
e r
ac
tio
n (
10
-2 m
ol/
g-h
)
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.22
0.24A
cid
ity
(m
mo
les
/g)
Figure 5. Comparison for acidity of the catalysts with therate of reaction.
Catalytic conversion of glycerol to ethers 903
carbonates can be improved using an adequate combi-
nation of a weak Lewis acid and Lewis base. In the
present case also, moderate Lewis acidic sites are gene-
rated with the impregnation of WO3 over SnO2. The
catalysts which possess most of these sites showed
maximum activity. The high acidity for the catalyst was
generated from Lewis acid sites which can be observed
from TPD of NH3 (figure 3). In order to understand
further how the surface and structural characteristics
are playing a role in glycerol etherification, the most
active catalysts was subjected to calcination at different
temperatures.
3.4b Effect of catalyst calcination temperature: The
catalyst 5WS was subjected to different calcination
temperatures in the range of 673 to 973 K and tested for
their etherification activity. The corresponding results
are shown in table 3. The conversion of glycerol
increased with increase in calcination temperature from
673 to 773 K and decreased thereafter. In order to find
the variation in activity with change in calcination tem-
perature, these catalysts were further characterized. The
acidity of the catalysts was measured from TPD of
ammonia. The acidity of the catalysts decreased gradu-
ally after calcination of the catalyst above 773 K. The
acidity of these catalysts and their etherification of glyc-
erol activity were correlated and the relevant plot is
shown in figure 5. The results indicate that there exists a
linear relation between acidity of the catalysts with that
of its glycerol etherification activity. Further, the in situ
Raman analysis supports the activity profiles of the cat-
alysts calcined at different temperatures. The catalysts
when exposed to temperature above 773 K showed the
bands related to crystalline α-WO3 phase. The growth
in the crystallites of WO3 resulted in the heterogeneity
on the catalysts surface at micrometer scale. This might
be a reason for low activity of the catalyst calcined
above 773 K. The conversion and selectivity during
glycerol etherification not only depends on the nature
Table 3. Comparison of glycerol conversion and product se-lectivity with 5WS catalyst calcined at different temperature.
Temperature (K) Conversion (%) Selectivity (%)
ME DE TE
673 70.1 93.3 4.3 2.3773 88.8 87.8 7.5 4.7873 81.5 92.6 4.3 2.8973 77.1 90.2 6.5 3.3
Reaction conditions: glycerol (1.84 g), tert-butanol (13.34 g),catalyst weight (0.5 g), reaction temperature (393 K), reac-tion time (1 h).
353 373 393 413 433
Co
nvers
ion
of
gly
cero
l (%
)
ME DE TE
Sele
cti
vit
y (
%)
Reaction temperature ( )
20
40
60
80
100
20
40
60
80
100
Figure 6. Comparison of glycerol conversion and productselectivity with reaction temperature. Reaction conditions:glycerol (1.84 g), tert-butanol (13.34 g), catalyst weight(0.5 g), reaction time (1 h).
of the catalyst but also on reaction parameters. The
influence of different reaction parameters such as reac-
tion temperature, catalyst weight and glycerol to tert-
butanol molar ratio were also studied.
3.4c Effect of reaction temperature: The etherifica-
tion of glycerol was carried out at different tempera-
tures ranging from 353 to 433 K and the results are
presented in figure 6. The percentage conversion of
glycerol increased with increase in reaction temperature
and attained an equilibrium at a reaction temperature
of 413 K. The selectivity also varied with increase in
reaction temperature. The formation of di and tri-tert-
butyl glycerol ethers was increased gradually with fur-
ther increase in reaction temperature. This is mainly due
to the attainment of maximum conversion of glycerol.
The successive etherification of ME is taking place at
high reaction temperature, as there was less availability
of glycerol. The optimum reaction temperature is 393K
as at this temperature maximum conversion and high
selectivity towards ME was obtained.
3.4d Effect of glycerol to tert-butanol molar ratio:
Figure 7 shows the effect of glycerol to tert-butanol
molar ratio on the etherification of glycerol. The con-
version of glycerol increased from 53.5 to 84% with the
increase in the molar ratio of glycerol to tert-butanol
from 1:3 and 1:6. Further increase in the molar does not
lead to appreciable increase in glycerol conversion. A
marginal decrease in glycerol conversion was observed
at a very high molar ratio of glycerol to tert-butanol of
1:12. The observed decrease in catalytic activity at high
molar ratio might be due to the saturation of catalyst
904 M Srinivas et al.
0
20
40
60
80
100
Conversion of glycerol
ME
DE
TE
Co
nvers
ion
of
gly
cero
l (%
)
1:121:91:61:3
Glycerol to tert-butanol mole ratio
0
20
40
60
80
100
120
Sele
cti
vit
y (
%)
Figure 7. Comparison of glycerol conversion and product selectivity withmole ratio. Reaction conditions: catalyst weight (0.5 g), reaction temperature(393 K), reaction time (1 h).
surface with tert-butanol by blocking the acidic centres.
Thus, one can say that there is a competitive adsorption
by tert-butanol on the acid sites with glycerol, which
reduces the efficiency of the catalyst. A glycerol to tert-
butanol molar ratio of about 1:6 to 1:9 was found to be
optimum for the selective synthesis of mono ethers with
high glycerol conversion.
3.4e Effect of catalyst loading: Catalyst loading is
an important parameter that needs to be optimized to
increase the glycerol conversion. Figure 8 shows the
effect of catalyst amount on glycerol conversion. From
the figure, it can be seen that the glycerol conver-
sion increased with increase in catalyst amount and
reached maximum at a catalysts amount of 21.4%.
5 10 15 20 25 30
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Conversion of glycerol
ME
DE
TE
Catalyst weight (%)
Ra
te o
f th
e r
ea
cti
on
(1
0-2 m
ol/
g-h
)
0
20
40
60
80
100
Sele
cti
vit
y (
%)
Figure 8. Comparison of glycerol conversion and product selectivity withcatalyst loading. Reaction conditions: glycerol (1.84 g), tert-butanol (13.34 g),reaction temperature (393 K), reaction time (1 h).
Catalytic conversion of glycerol to ethers 905
Thereafter, further increase in catalyst loading no sub-
stantial increase in glycerol conversion was observed.
However, as more catalyst is introduced into the reac-
tion mixture, it provides an external contact surface area
that facilitates further etherification of mono-tert-butyl
glycerol ether. As more ME is produced, it eventually
acted as co-solvent and subsequently, the reaction rate is
being controlled by the diffusion of the reactants to the
active sites, instead of catalyst loading. Once, increas-
ing the dosage of catalyst in the reaction mixture had
no significant effect on the glycerol conversion. How-
ever, with increase in the catalyst amount the selectivity
towards DE and TE have been increased. This is mainly
because there was less availability of glycerol, as it is
1 2 3 40
20
40
60
80
100
Co
nvers
ion
(%
)
Number of cycles
Conversion of glycerol
0
20
40
60
80
100
120 Selectivity
Sele
cti
vit
y (
%)
Figure 9. Reusability of 5 wt% WO3/SnO2 catalyst foretherification of glycerol. Reaction conditions: glycerol (1.84g), tert-butanol (13.34 g), catalyst weight (0.5 g), reactiontemperature (393 K), reaction time (1 h).
completely converted into ME, the available acid sites
favours the secondary etherification of mono tertiary
butyl glycerol ethers to HE.
3.4f Reusability of the catalyst: The catalyst was
used repeatedly for etherification of glycerol to inves-
tigate its reusability. The used catalyst was recovered
from the reaction mixture and washed with methanol,
dried and calcined at 773K for 2 h. This catalyst is
reused for etherification of glycerol to study its perfor-
mance and the results are shown in figure 9. The cata-
lyst was repeatedly used for 4 reaction cycles. The cat-
alyst activity did not change appreciably with cycles
of reuse. The conversion of glycerol and selectivity
towards mono- tert-butyl glycerol ether were same for
the used catalysts as that of the fresh catalyst. XPS
measurements were made on the 5WS catalyst before
and after reaction and relevant spectra are shown in
figure 10. The XPS spectra of Sn 3d showed two peaks
at 486.8 and 495.2 eV corresponding to Sn 3d5/2 and
Sn 3d3/2, respectively. These values correspond to that
of Sn (IV) oxide.52 The photoelectron peaks of W 4f7/2
and W 4f5/2 appeared at 35.9 and 37.9 eV which are
identical to that of W (VI) oxide.52 The XPS results
revealed the existence of both WO3 and SnO2 in their
simple oxide forms and that there was no formation of
any mixed oxides.
3.4g Synthesis of glycerol carbonate: The high
active catalyst of the etherification of glycerol prompted
to study for the carbonylation of glycerol to yield glyc-
erol carbonate. The carbonylation of glycerol with urea
was performed with our previously repeated reaction
Figure 10. XPS spectrum of the elements present in the fresh and used catalysts (a) O (b) Sn and (c) W.
906 M Srinivas et al.
2.5 5.0 7.5 10.0
0
20
40
60
80
100
Sele
cti
vit
y (
%)
WO3 loadings (%)
by-product
glycerol carbonate
0
10
20
30
40
50
Co
nvers
ion
of
gly
cero
l (%
)
Figure 11. Catalytic performance of x WS catalysts for thesynthesis of glycerol carbonate. Reaction conditions: glyc-erol (2 g), urea (1.3 g), catalyst weight (0.2 g), reactiontemperature (413 K), reaction time (4 h).
procedure. The activity profiles for the synthesis of
glycerol carbonate from glycerol and urea are presented
in figure 11. In the glycerol carbonylation with urea,
the main product was glycerol carbonate. The other by-
products are 4-(hydroxymethyl) oxazolidine-2-one and
(2-oxo-1, 3-dioxolan-4-yl) methyl carbamate. In the
present study, only (2-oxo-1, 3-dioxolan-4-yl) methyl
carbamate was observed as by-product. In the synthe-
sis of glycerol carbonate also the 5WS catalysts showed
better activity. About 40.4% of glycerol conversion
with 85.4% selectivity towards glycerol carbonate was
achieved over this catalyst. These results indicate that
the WO3 supported on SnO2 are highly active solid
acid catalysts for the conversion of glycerol into value
added chemicals such as glycerol ethers and glycerol
carbonate.
3.4h Comparison of the catalyst with other reported
catalysts: Table 4 presents etherification of glycerol
Table 4. Comparison of glycerol etherification activity ofpresent catalysts with other reported catalysts.
S. No Catalysts Time (min) Conversion (%) Ref
1 5WS 60 84 [Present work]
2 SCC-S 240 81.6 62
3 A-35 480 86 63
4 A-15 480 79 63
5 5WSa 240 40.4 [present work]
6 SnW21a 240 52.1 64
a These catalysts were used for synthesis of glycerol carbonate
with tert-butanol found in the literature compared with
results obtained in this work. By analyzing the table,
it was possible to confirm that the results obtained
using 5WS as a catalyst for etherification of glyc-
erol with tert-butanol were quite satisfactory because
the conversion were similar or higher than other cat-
alysts presented in the literature.62,63 The use of cata-
lysts such as acid ion exchange resins (A-15 and A-
35) and sulfated carbon catalysts (SCC-S) showed the
reasonable conversion of glycerol and also showed the
formation of tert-butyl ethers as by-product at higher
temperatures. The best results were obtained with the
pure isobutylene but it is very expensive. Additionally,
these other catalysts had production costs much higher
than present 5WS catalyst prepared from low-cost raw
materials.
The present 5WS catalysts was also compared for its
carbonylation of glycerol to glycerol carbonate activ-
ity with our previous reported tin-tungsten mixed oxide
(SnW21) catalyst.64 SnW21 catalyst showed about
52.1% conversion of glycerol whereas the 5WS catalyst
showed 40.4% under similar conditions. The impor-
tant thing about present 5WS catalyst is that it showed
activity in both carbonylation and etherification of glyc-
erol, whereas SnW21 showed better activity only in
carbonylation reaction.
4. Conclusions
Tungsten oxide supported on tin oxide catalysts are
highly active for the conversion of glycerol into its
ethers and glycerol carbonate. The catalytic activity
depended on the acidity of the catalysts which in turn
is related to the presence of well dispersed amorphous
WO3. The nature of WO3 species depended on the
amount of WO3 and the catalyst calcination tempera-
ture. The catalyst with 5 wt.% WO3 on SnO2 calcined
at 773 K for 4 h exhibited maximum activity. The con-
version and selectivity during the glycerol etherifica-
tion also depended on the reaction temperature, cat-
alyst, concentration and molar ratio of reactants and
these parameters have been optimized.
Acknowledgements
MS and GR thank Council of Scientific and Industrial
Research (CSIR), India for the financial support in the
form of a Senior Research Fellowship. We also thank
Ms. Marline Daniel for Raman spectral analysis. NL
thanks CSIR for the award of Raman Research Fellow-
ship to carry part of the present work at IRCELYON,
France.
Catalytic conversion of glycerol to ethers 907
References
1. Corma A, Sara I and Alexandra V 2007 Chem. Rev. 1072411
2. Chun-Hui (Clayton) Z, Beltramini J N, Yong-Xian F andLu(Max) G Q 2008 Chem. Soc. Rev. 37 527
3. Behr A, Eilting J, Irawadi K, Leschinski J and LindnerF 2008 Green Chem. 10 13
4. Hirai T, Ikenaga N, Miyake T and Suzuki T 2005 EnergyFuels 19 1761
5. Balaraju M, Rekha V, Sai Prasad P S, Prabhavathi DeviB L A, Prasad R B N and Lingaiah N 2009 Appl. Catal.A: Gen. 354 82
6. Climent M J, Corma A, Frutos P D, Sara I, Maria N,Alexandra V and Concepción P 2010 J. Catal. 269140
7. Katryniok B, Paul S, Capron I, Christine L, Bellière-Baca V, Patrick R and Franck D 2010 Green Chem. 121922
8. Dam J, Djanashvili K, Kapteijn F and Hanefeld U 2013ChemCatChem 5 497
9. Joel B and François J 2008 Eur. J. Lipid Sci. Technol.110 825
10. Rahmat N, Abdullah A Z and Mohamed A R 2010Renew. Sust. Energ. Rev. 14 987
11. Karinen R S and Krause A O I 2006 Appl. Catal A: Gen.306 128
12. Kovvali A S and Sirkar K K 2002 Ind. Eng. Chem. Res.41 2287
13. Parameswaram G, Srinivas M, Hari Babu B, Sai PrasadP S and Lingaiah N 2013 Catal. Sci. Technol. 3 3242
14. Selva M and Fabris M 2009 Green Chem. 11 116115. Ghandi M, Mostashari A, Karegar M and Barzegar M
2007 J. Am. Oil Chem. Soc. 84 68116. Rokicki G, Rokoczy P, Parzuchowski P and Sobiecki M
2005 Green Chem. 7 52917. Teles J H, Rieber N and Harder W 1994 US Patent
535909418. Malyaadri M, Jagadeeswaraiah K, Sai Prasad P S and
Lingaiah N 2011 Appl. Catal. A: Gen. 401 15319. Simanjuntak F S H, Kim T K, Lee S D, Ahn B S,
Kim H S and Lee H 2011 Appl. Catal. A:Gen. 401220
20. Takagaki A, Iwatani K, Nishimura S and Ebitani K 2010Green Chem. 12 578
21. Climent M J, Corma A, Frutos P D, Iborra S, NoyM, Velty A and Concepcion P 2010 J. Catal. 269140
22. Hammond C, Sanchez J A L, Rahim M H A, DimitratosN, Jenkins R L, Carley A F, He Q, Kiely C J, Knight DW and Hutchings G J 2011 Dalton Trans. 40 3927
23. Marcos F R, Casilda V C, Banares M A and FernandezJ F 2010 J. Catal. 275 288
24. Dibenedetto A, Angelini A, Aresta M, Ethiraj J, FragaleC and Nocito F 2011 Tetrahedron 67 1308
25. Vieville C, Yoo J W, Pelet S and Mouloungui Z 1998Catal. Lett. 56 245
26. George J, Patel Y, Pillai S M and Munshi P 2009 J. Mol.Catal. A: Chem. 304 1
27. Yanlong G, Azzouzi A, Pouilloux Y, François J andBarrault J 2008 Green Chem. 10 164
28. González M D, Cesteros Y and Salagre P 2013 Appl.Catal. A: Gen. 450 178
29. Frusteri F, Arena F, Bonuraa G, Cannilla C, SpadaroaL and Di Blasi O 2009 Appl. Catal.A: Gen. 36777
30. Yuan Z, Xia S, Chen P, Hou Z and Zheng X 2011 EnergyFuels 25 3186
31. Behr A and Obendorf L 2002 Eng. Life. Sci. 2 18532. Klepacova K, Mravec D, Kaszonyi A and Bajus M 2007
Appl. Catal. A: Gen. 328 133. Melero J, Vicente A G, Morales G, Paniagua M, Moreno
J M, Roldan R, Ezquerro A and Perez C 2008 Appl.CatalA: Gen. 346 44
34. Klepacova K, Mravec D, Hajekova E and Bajus M 2003Pet. Coal 45 54
35. Janaun J and Ellis N 2010 J. Appl. Sci. 10 263336. Viswanadham N and Saxena S K 2012 Fuel 105 49037. Yadav G D, Chandan P A and Gopalaswami N 2012
Clean. Techn. Environ. Policy 14 8538. Lupan O, Chow L, Chai G, Schulte A, Park S and
Heinrich H 2009 Mater. Sci. Eng. B 157 10139. Okutsu T 2007 J. P. Pat. 03934740. Yoo J W and Mouloungui Z 2003 Stud. Surf. Sci. Catal.
146 75741. Park J H, Choi J S, Woo S K, Lee S D, Cheong M, Kim
H S and Lee H 2012 Appl. Catal. A: Gen. 433 3542. Aresta M, Dibenedetto A, Nocito F and Ferragina C
2009 J. Catal. 268 10643. Climent M J, Corma A, Frutos P D, Iborra S, Noy M,
Velty A and Concepcion P 2010 J. Catal. 269 14044. Marcos F R, Casilda V C, Banares M A and Fernandez
J F 2010 J. Catal. 275 28845. Ramesh Kumar Ch, Jagadeeswaraiah K, Sai Prasad P S
and Lingaiah N 2012 ChemCatChem 4 136046. Hammond C, Sanchez J A L, Rahim M H A,
Dimitratos N, Jenkins R L, Carley A F, He Q, Kiely C J,Knight D W and Hutchings G J 2011 Dalton Trans. 403927
47. Furuta S, Matsuhashi H and Arata K 2004 Appl. Catal.A: Gen. 269 187
48. Maksimov G M, Litvak G S, Budneva A A, Paukshtis EA, Salanov A N and Likholobov V A 2006 Kinet. Catal.47 564
49. Mallesham B, Sudarsanam P, Raju G and Reddy B M2013 Green Chem. 15 478
50. Shoulia B, Dianqinga L, Dongmeia H, Ruixiana L,Aifana C and Liub C C 2010 Sens. Actuators B 150749
51. Ayyappan S, Subbanna G N and Rao C N R 1995 Chem.Eur. J. 1 165
52. Sarkar A, Ghosh S K and Pramanik P 2010 J. Mol.Catal. A: Chem. 327 73
53. Loridant S 2002 J. Phys. Chem. B 106 1327354. Abello L, Bochu B, Gaskov A, Koudryavtseva S,
Lucazeau G and Roumyantseva M 1998 J. Solid StateChem. 135 78
55. Scheithauer M Grasselli R K and Knözinger H 1998Langmuir 14 3019
56. Ross-Medgaarden E I and Wachs I E 2007 J. Phys.Chem. C 111 15089
57. Loridant S, Feche C, Essayem N and Figueras F 2005 J.Phys. Chem. B 109 5631
58. Boulova M and Lucazeau G 2002 J. Solid State Chem.167 425
908 M Srinivas et al.
59. Kim T, Burrows A, Kiely C J and Wachs I E 2007 J.Catal. 246 370
60. Ramu S, Lingaiah N, Prabhavathi Devi B L A, PrasadR B N, Suryanarayana I and Sai Prasad P S 2004 Appl.Catal. A: Gen. 276 163
61. Ball P, Fullmann H and Heitz W 1980 Angew. Chem.Int. Ed. Engl. 19 718
62. Gonçalves M, Souza V C, Galhardo T S, Mantovani M,Figueiredo F C A, Mandelli D and Carvalho W A 2013Ind. Eng. Chem. Res. 52 2832
63. Klepácova K, Mravec D and Bajus M 2006 Chem. Pap.60 224
64. Jagadeeswaraiah K, Kumar C R, Prasad P S S, LoridantS and Lingaiah N 2014 Appl. Catal. A: Gen. 469 165